feat: Render 3D cube with camera controls (zoom, pan, rotate)

Co-authored-by: aider (gemini/gemini-2.5-pro) <aider@aider.chat>

src/MainWindow.cpp

@@ -1,7 +1,7 @@
 #include "MainWindow.h"
+#include "ViewportWidget.h"
 
 #include <QToolBar>
-#include <QWidget>
 
 MainWindow::MainWindow(QWidget *parent)
     : QMainWindow(parent)
@@ -11,7 +11,6 @@ MainWindow::MainWindow(QWidget *parent)
 
     QToolBar *toolBar = addToolBar("Tools");
 
-    QWidget *viewport = new QWidget;
-    viewport->setStyleSheet("background-color: darkgray;");
+    ViewportWidget *viewport = new ViewportWidget;
     setCentralWidget(viewport);
 }